Method of reprocessing residues containing dihydroxy compounds

ABSTRACT

A process for working up residues containing dihydroxy compounds resulting from the preparation of polyesters, where 
     (1) in a first stage there is esterification or transesterification of a dicarboxylic acid or esters thereof or ester-forming derivatives with a molar excess of a dihydroxy compound, 
     (2) in at least one second stage there is polycondensation of the esterification product obtained in (1), 
     (3) the vapors (a) and (b) resulting from the reactions in (1) and (2) respectively are subjected to a treatment to recover the starting materials, 
     wherein the treatment to recover the starting materials is carried out in the presence of an alkali metal or alkaline earth metal compound in an amount of from 0.5 to 10% by weight, calculated as alkali metal or alkaline earth metal, based on the solids content of the vapors.

The present invention relates to a process for working up residuescontaining dihydroxy compounds resulting from the preparation ofpolyesters, where

(1) in a first stage there is esterification or transesterification of adicarboxylic acid or esters thereof or ester-forming derivatives with amolar excess of a dihydroxy compound,

(2) in at least one second stage there is polycondensation of theesterification product obtained in (1),

(3) the vapors (a) and (b) resulting from the reactions in (1) and (2)respectively are subjected to a treatment to recover the startingmaterials.

Polyesters, especially polyalkylene terephthalates, are prepared on alarge scale in transesterification/polycondensation processes where anesterification or transesterification is carried out in a first stage,and the actual polycondensation is carried out in at least one furtherstage (cf. Chemiefasern/Textilindustrie 40 (1992), 1058-1062 andUllmann's Enzyklop{umlaut over (a)}dieder technischen Chemie, 4thEdition, Volume 19, pages 61-88).

This process will be briefly explained taking the example of thepreparation of polybutylene terephthalate from terephthalic acid and1,4-butanediol.

In a first reaction chamber, terephthalic acid is esterified with amolar excess, preferably 50-120 mol %, in particular 70-100 mol %, of1,4-butanediol, and the esterified compound is subjected in furthersteps to the actual polycondensation. The vapors resulting from theesterification are transferred into a column in which the low-boilingcomponents THF/water are removed as distillate, and a bottom productwhich, besides excess 1,4-butanediol, also contains small amounts ofoligomers, polymers and terephthalic acid is obtained.

The esterification product is subsequently polycondensed, expediently inat least two stages, called the precondensation and postcondensation, incontinuous processes.

For economic reasons, it is desirable in this connection that as much aspossible of the resulting reaction products and dihydroxy compoundspresent in excess are treated further in order, for example, to recoverthe 1,4-butanediol and generate as little waste as possible.

A process for treating the vapors from the polycondensation is proposedin DE-A 19509957.5 which is not a prior publication.

DE-A 43 33 929 discloses a process for recovering the startingmaterials, where a distillation residue consisting ofdihydroxy-containing compounds (called “Hex”) is metered into the columnto simplify manipulation of the bottom products which are mostlyextremely viscous.

The disadvantage of this process is that this residue usually derivesfrom the distillation of butanediol or hexanediol and is thus notavailable everywhere.

Since the vapors contain oligomeric and polymeric esters, in addition tothe starting materials and byproducts, the pH of the vapors is below 7owing to the content of carboxyl end groups. In addition, renewedfragmentation takes place during recovery, resulting in new carboxyl endgroups besides those already present. For these reasons, treatment ofthe vapors is very costly because it is necessary to employcorrosion-resistant stainless steel containers.

During the treatment and preparation of polyesters there is considerableformation of, for example, THF as byproduct (when 1,4-butanediol is thestarting material). Since this side reaction is catalyzed by acids,there are also considerable losses of 1,4-butanediol due to THFformation on recovery by distillation.

It is an object of the present invention to remedy the disadvantagesdescribed above and to improve the working up of the vapors so that mostof the dihydroxy compounds present therein can be recovered aseconomically as possible. It is intended at the same time that thequality of the polyester product be maintained.

We have found that this object is achieved by carrying out the treatmentto recover the starting materials in the presence of an alkali metal oralkaline earth metal compound in an amount of from 0.5 to 10% by weight,calculated as alkali metal or alkaline earth metal, based on the solidscontent of the vapors.

The addition, according to the invention, of alkali metal or alkalineearth metal compounds in the treatment of the vapors results in a basicbuffering of the vapors so that corrosion of the containers is no longerpossible. This makes it possible to employ considerably lower-costordinary steel containers for these purposes. In addition, the formationof byproducts, eg. THF, is considerably reduced.

Moreover, at a pH of 7 or above for the vapors, shorter fragments, eg.of the polymeric residues, are present so that, on the one hand, theycan be dispersed more easily in the dihydroxy compound, ie. theresulting distillation residue is easier to handle because it has alower viscosity. On the other hand, the columns can be operated forconsiderably longer without cleaning (longer service lives) and thesolid residues can be removed more easily from the column on cleaning,eg. with water.

In a particular embodiment of the invention, further butanediol, forexample, can be recovered in the evaporation apparatus following thedistillation treatment, and this working up is independent of theavailability of the residue containing dihydroxy compounds (HEX).

In the process according to the invention, from 0.5 to 10, preferably 1to 8 and, in particular, 2.5 to 6, % by weight, calculated as alkalimetal or alkaline earth metal, based on the solids contents of thevapors, are added preferably to the bottom product from columns A, B orC in the treatment of the vapors.

Suitable in principle are inorganic and organic compounds of the alkalimetals, preferably of Li, Na, K, particularly preferably Na compounds.

Suitable inorganic compounds of the alkaline earth or alkali metals,preferably of sodium, are, for example, the corresponding silicates,phosphates, phosphites, sulfates or, preferably, carbonates,bicarbonates and hydroxides.

The organic compounds of the alkaline earth or alkali metals, preferablyof sodium, include the corresponding salts of (cyclo)aliphatic,araliphatic or aromatic carboxylic acids having, preferably, up to 30carbon atoms and, preferably, 1 to 4 carboxyl groups. Examples thereofare: alkali metal salts of formic acid, acetic acid, propionic acid,butyric acid, isobutyric acid, caprylic acid, stearic acid,cyclohexanecarboxylic acid, succinic acid, adipic acid, suberic acid,1,10-decanedicaroxylic acid, 1,4-cyclohexanedicarboxylic acid,terephthalic acid, 1,2,3-propanetricarboxylic acid,1,3,5-cyclohexanetricarboxylic acid, trimellitic acid,1,2,3,4-cyclopentanetetracarboxylic acid, pyromellitic acid, benzoicacid, substituted benzoic acids, dimer acid and trimer acids, andneutral or partially neutralized montan wax salts or montan wax estersalts (montanates). Salts with other types of acid residues, such asalkali metal paraffin-, olefin- and arylsulfonates or else phenolates,and alcoholates, such as methanolates, ethanolates, glycolates, can alsobe employed according to the invention. Sodium carbonate, sodiumbicarbonate, sodium hydroxide, sodium salts of mono- and polycarboxylicacids, in particular aliphatic mono- and polycarboxylic acids,preferably those having 2 to 18 carbon atoms, in particular having 2 to6 carbon atoms, and up to four, preferably up to two, carboxyl groups,and sodium alcoholates having, preferably, 2 to 15, in particular 2 to8, carbon atoms are preferably used. Examples of particularly preferredrepresentatives are sodium acetate, sodium propionate, sodium butyrate,sodium oxalate, sodium mallonate, sodium succinate, sodium methanolate,sodium ethanolate, sodium glycolate. Sodium methanolate is veryparticularly preferred and is particularly advantageously employed in anamount of from 2.5 to 6% by weight, calculated as Na, based on thesolids content of the vapors. It is also possible to employ mixtures ofvarious alkaline earth or alkali metal compounds.

The alkaline earth or alkali metal or compound thereof can be added inat least one column A, B or C depending on the method of working up. Itis, of course, also possible to distribute the addition over all thecolumns, in which case it is preferred to meter equal amounts into eachcolumn.

The process according to the invention will be illustrated hereinafterby the example once again of the preparation of polybutyleneterephthalate with reference to the figures; however, it is emphasizedonce again that it is also suitable correspondingly for preparing otherpolyesters known to the skilled worker.

Firstly, terephthalic acid and 1,4-butanediol (the latter in an excessof from 150 to 220 mol %, preferably 70 to 100 mol %) are reactedtogether in a conventional way at from 150 to 220° C. under from 0.7 go2.0 bar for from 150 to 300, preferably 200 to 280, minutes, duringwhich esterification takes place and resulting THF (tetrahydrofuran) istransferred together with excess butanediol (BD) and small amounts ofoligomeric and polymeric compounds, and residual amounts of terephthalicacid, with the vapors (a) into a column A. The feed point is preferablyin the middle or in the lower part of the column.

Stage (1) of the process is depicted in the figures as one process step.Stage (1) is preferably divided into at least four process steps, thesebeing composed of a mixing reactor for solvent, catalyst etc. and atleast three stirred reactors.

The esterification product from stage (1) of the process ispolycondensed in at least one second stage (2). This polycondensation iscarried out in a conventional way at from 240° C. to 270° C. under from0.3 to 200 mbar for from 60 to 200, preferably 70 to 180, minutes. Aparticularly preferred embodiment of stage (2) is division into twoprocess steps, carrying out firstly a precondensation and then apostcondensation.

FIG. 1 depicts a preferred embodiment for recovering the startingmaterials, in which the vapors (a) resulting in stage (1) aretransferred into at least one column A, and the low-boiling componentsof the vapors (a) are removed as distillate, and the bottom product isreturned to stage (1), and the vapors (b) resulting in stage (2) aretransferred into at least one column B, and the low-boiling componentsare removed as distillate, and the bottom product is discharged fromcolumn B and subsequently subjected in at least one column C to afurther treatment to recover the dihydroxy compound.

FIG. 2 depicts a further preferred process for working up the vapors, inwhich the vapors (a) resulting in stage (1) and part of the vapors (b₁)from stage (2) of the process are combined in at least one column A, andthe low-boiling components of the vapors are removed as distillate incolumn A, and the bottom product which mainly contains the excessdihydroxy compounds, and oligomeric and polymeric reaction products, isreturned to stage (1), and the other part of the vapors (b₂) from stage(2) of the process is transferred into at least one column B, and thelow-boiling components of the vapors are removed as distillate, and thebottom product is discharged from column B and subsequently subjected inat least one column C to a further treatment to recover the dihydroxycompound.

It is possible by dividing the vapors from the polycondensation, withpreferably the vapors (b₁) deriving from the precondensation and thevapors (b₂) deriving from the postcondensation, in a technically simplemanner to return the excess diol to the esterification. This makes theprocess more economic and less costly, while the quality of thepolyester product is maintained.

FIG. 3 depicts another preferred embodiment of the process, in which thevapors (a) and (b) resulting in stages (1) and (2) are transferred intoat least one column A, and the low-boiling components are removed asdistillate, and subsequently the bottom product is discharged fromcolumn A and subjected in at least one column C to a further treatmentto recover the dihydroxy compound.

The low-boiling THF/water is removed as distillate in column A or B ofthe process according to the invention, and these low-boiling componentsare subsequently subjected to a recovery of the starting materials(separation of THF from water).

In the case of the preparation of polyethylene terephthalate, thelow-boiling components essentially consist of water or methanol withacetaldehyde; in the preparation of PBT starting from dimethylterephthalate they consist of methanol and THF. The bottom product,which mainly contains the excess dihydroxy compounds, is then returnedto stage (1). Depending on the size of the system, columns A to C can bedivided into a plurality of columns A₁, A₂ to A_(n) or B₁, B₂ to B_(n)and C₁, C₂ to C_(n).

In the preferred embodiments depicted in FIGS. 2 and 3, the vapors areworked up in such a way that the vapors from stage (2), which mainlycontain THF, water, excess butanediol and somewhat larger amounts ofoligomeric and polymeric compounds than in the vapors from stage (1),and residual amounts of terephthalic acid, are transferred either partlyinto columns A and B or completely into a column A, in which case thevapors from stage (1) of the process are already present in column A.

In the preferred embodiment in FIG. 1, the working up takes placeseparately and the vapors (b) are transferred into a column B.

The feed point is preferably in the middle or in the lower part of thecolumns. The manner of dividing up these parts is self-evident becausethe dividing up depends on the capacity of the columns and the qualityof the required starting materials in the esterification.

The low-boiling components of the vapors are removed as distillate, andthe bottom product from column A and/or B, depending on the procedure,is discharged. The bottom product is subsequently subjected to a furthertreatment to recover the dihydroxy compound. This is done by dischargingthe bottom product from column A or B, depending on the procedure, intoa column C, with or without removal of the solids.

A liquid residue containing dihydroxy compounds (“Hex”), resulting, forexample, from the distillation of 1,4-butanediol or 1,6-hexanediol ispreferably introduced into column C in parallel. There is no particularrestriction per se on the composition of the residue as long as it isliquid and contains no compounds interfering with the separation in thecolumn. This is usually the case with residues from the distillation ofbutanediol or hexanediol.

The feed point is preferably in the middle or in the lower part of thecolumn, and the feed rate is generally from 0.03 to 5 kg per kg ofbottom products transferred into the column, preferably from 0.04 to 0.1kg/kg.

The addition of this residue containing dihydroxy compounds results inthe bottom product in column C remaining liquid or transportable, and itis therefore possible to obtain in a technically simple manner thedihydroxy compound as distillate and a bottom product in this column,which is once again liquid or transportable and can easily be deliveredfor incineration.

The dihydroxy compound is subsequently returned from column C to theesterification (stage 1).

In a particularly preferred embodiment of the process according to theinvention, the bottom product from column C is discharged into anevaporation apparatus D, and the dihydroxy compound is removed andreturned to column C, and the remaining residue is discharged from D.

This preferred embodiment results in further 1,4-butanediol, forexample, being recovered and makes it possible to work up the residue ina simple and economic manner without adding a distillation residue.Another advantage of this procedure is that it is unnecessary forremoval of the dihydroxy compound in column C to be complete, and theresidue is not so viscous and thus can still be handled.

The feed point of the recovered butanediol is preferably in the lowerpart of column C, in particular above the vapor space of the bottom ofcolumn C.

Examples of suitable evaporation apparatus D are special columns,stirred evaporators, extruders, self-cleaning evaporators and trayevaporators.

The temperature of the evaporation apparatus is preferably above themelting point of the remaining residue. This depends on the solidscontent and the nature of the residue. The temperature for the residuefrom the preparation of polybutylene terephthalate is normally 180 to250° C., preferably 190 to 230° C., and for residues from thepreparation of polyethylene terephthalate is generally from 180 to 280°C., preferably 190 to 265° C.

If the bottom product is concentrated in apparatus D only to a solidscontent of from 30 to 70%, based on the discharged bottom product, lowertemperatures from 150 to 220° C. in apparatus D are likewise suitable.

It is possible via the process according to the invention to recover thestarting materials such as butanediol to a considerable extent in asimple and low-cost manner. At the same time, the content of byproductssuch as THF is drastically reduced.

EXAMPLE 1 (FIG. 2)

Stage 1

35.2 mol of terephthalic acid (TPA) and 82.7 mol of 1,4-butanediol (BD)were reacted in a 3-stage esterification cascade at 210-220° C. under 2bar absolute. 30 ppm Ti based on polybutylene terephthalate (PBT) wereadded as tetrabutyl orthotitanate (TBOT) as catalyst (residence timeτ=205-260 min in total).

The reaction product was transferred with a conversion of at least 95%into stage 2. At the start of the precondensation, 50 ppm Ti based onPBT were added as TBOT:

τ=30-40 min,

T=240-290° C.,

P=0.8 bar initially up to 25 mbar.

After the precondensation in stage 2, the product was transferred with aconversion of at least 98.5% into the postcondensation of stage 2. Atthe start of the postcondensation, 30 ppm Ti based on PBT were added asTBOT.

τ=90-140 min,

T=245-265° C.,

P=0.5 to 2,0 mbar.

The PBT product was then discharged as a melt with a viscosity η(intrinsic) of 0.9-1.3 and was granulated.

The excess molar BD was worked up as depicted in principle in FIG. 2under the various conditions shown in the table.

Procedure for BD Recovery:

The vapors from stage (1) were transferred as stream (a) (see Tab. toFIG. 2) at 210-220° C. under 2 bar absolute into column A. Water and THFwere removed as distillate. The butanediol was returned to stage (1).The butanediol present in the vapors from stage (2) (stream b₁+b₂) wasfed as liquid with all the vapors from this stage into columns A and B.This entailed the pre- and postcondensation streams being divided sothat the vapors from the postcondensation in stage (2) and part of thevapors from the precondensation in stage (2) being fed as liquid intocolumn B, while the other part of the vapors from the precondensation instage (2) was fed into column A.

Column A was operated under the same pressure as the esterification instage (1). Columns/apparatus B, C, D were operated under a pressure ≦1bar absolute. THF and water were taken off as distillate from column B,and the butanediol present in the bottom product from column B was fedinto column C. Butanediol was removed as distillate from column C. Thebottom product from column C was fed into apparatus D where thebutanediol was removed quantitatively as vapor and returned to the vaporspace over the bottom of column C.

With the procedure of Example 1, 0.1 mol of PBT was discharged per 35.2mol of TPA from apparatus D. Without apparatus D, the same amount ofsolid was discharged from the bottom of column C.

The BD losses for the various working-up variants of Example 1 arelisted in Table 1 for FIG. 2.

Working-up variants 1.1. to 1.5. therein were carried out in thefollowing ways:

1.1. The distillation residue “Hex” was fed into column B in a totalamount of 3.4 kg/kg of solids present in vapors b₂ (for comparison asdisclosed in DE-A 43 33 929).

1.2. Sodium methanolate (30% strength in methanol) was fed into column Bin an amount of 0.05 kg Na/kg of solids present in vapors b₂.

In addition, “Hex” was fed in as in 1.1.

1.3. Na methanolate was fed in as in 1.2. but without “Hex”. The bottomproduct from column C was fed into evaporation apparatus D. The BD wasreturned as vapor to the vapor space above the bottom of column C. Thesolid was discharged from D as melt at 235° C.

1.4. Neither “Hex” nor Na methanolate was fed in (for comparison).

1.5. As in 4 but without apparatus D (for comparison).

TABLE 1 Example 1 X Service VIII life III VII BD loss in Heat I BD BDfor the IX exchangers THF II Dis- IV V working working up Solid ColumnsWorking-up Reac- THF charge b₂ b₂ b₁ b₁ VI up (THF + discharge Pipelinesvariant tion Distill. C or D back in back in a a + b₁ + b₂ discharge) Cor D (in days) 1.1. *) 7.0 0.5 0.6 7.5 8.6 17.9 17.9 14.1 40.6 1.1 0.115-20 1.2. 7.0 <0.1 0.3 8.3 8.6 17.9 17.9 14.1 40.6 0.3 0.1  30-200 1.3.7.0 <0.1 <0.1 8.6 8.6 17.9 17.9 14.1 40.6 <0.1 0.1  30-150 1.4. *) 7.00.75 0.3 7.55 8.6 17.9 17.9 14.1 40.6 1.05 0.1  7-10 1.5. *) 7.0 0.751.2 6.65 8.6 17.9 17.9 14.1 40.6 1.95 0.1  7-10 I—IX: all data in molIX: one building block of the PBT polymer of diol and acid (molecularweight: 220 g) counts as mol I: THF formation in stages 1 and 2 II: THFformation during working up III: BD in the bottom product dischargedfrom C and D IV: b₂ (in): BD in vapors b₂ b₂ (back): BD recovered fromb₂ V: b₁ (in + back): BD in vapors b₁ (completely recovered) VI: a: BDin vapors a (completely recovered) VII: all the BD fed into the workingup VIII: total loss of BD in the working up IX: solid discharged from Cand D (0.1 mol of PBT based on 35.2 mol of TPA employed) *) forcomparison

EXAMPLE 2 (FIG. 1)

Stage 1

35.2 mol of dimethyl terephthalate (DMT) and 54.0 mol of BD were reactedin a 3-stage transesterification cascade at 180-205° C. under 1.16 barabsolute. As catalyst, 115 ppm Ti based on PBT were added as TBOT and0.7 mmol of Na methanolate per kg of PBT at the start of the reaction(residence time τ=205-260 min in total.

The reaction product was transferred with a conversion of at least 95%into stage 2. The precondensation was carried out in the first part ofstage 2:

τ=30-40 min,

T=240-290° C.,

P=0.8 bar initially up to 25 mbar.

After the precondensation in stage 2, the product was transferred intothe postcondensation in stage 2:

τ=90-140 min,

T=245-265° C.,

P=0.5-2.0 mbar.

The product was then discharged as a melt with a viscosity η (intrinsic)of 0.9-1.3 and was granulated.

The vapors were worked up as depicted in principle in FIG. 1 under thevarious conditions shown in Table 2.

Working-up variants 2.1. to 2.5. listed in the table were carried outunder similar conditions to those listed under 1.1. to 1.5.

As a difference from the working up in Example 1, all the vapors fromstage 2 were fed into column B. As a difference from Example 1, MeOH andTHF were removed as distillate from columns A and B.

The conditions of pressure, temperature, “Hex”, Na and solids contentcorresponded to Example 1.1 to 1.5.

TABLE 2 Example 2 IX Service VII life VI BD loss in Heat III BD the VIIIexchangers I II BD IV working working up Solid Columns Working-up THFTHF Discharge b b V up (THF + dis- discharge Pipelines variant ReactionDistill. C or D back in a a + b charge) C or D (in days) 2.1. *) 3.5 0.50.6 7.3 8.4 7.1 15.5 1.1 0.1 15-20 2.2. 3.5 <0.1 0.3 8.1 8.4 7.1 15.50.3 0.1  30-200 2.3. 3.5 <0.1 0 8.4 8.4 7.1 15.5 <0.1 0.1  30-150 2.4.*) 3.5 0.75 0.3 7.35 8.4 7.1 15.5 1.05 0.1  7-10 2.5. *) 3.5 0.75 1.26.45 8.4 7.1 15.5 1.95 0.1  7-10 I-VIII: all data in mol VIII: onebuilding block of the PBT polymer of diol and ester (molecular weight:220 g) counts as mol I: THF formation in stages 1 and 2 II: THFformation during working up III: BD in the bottom product dischargedfrom C and D IV: b (in): BD in vapors b b (back): BD recovered from b V:a: BD in vapors a (completely recovered) VI: all the BD fed into theworking up VII: total loss of BD in the working up VIII: soliddischarged from C and D (0.1 mol of PBT produced per 35.2 mol of DMTemployed) *) for comparison

EXAMPLE 3 (FIG. 3)

The reaction took place as in Example 2 with the difference that all thevapors from stages 1 and 2 were fed into column A. MeOH and THF wereremoved as distillate from this column. The bottom product wastransferred into column C. Further working up took place as in Example2. The statements concerning “Hex”, Na and D apply correspondingly.“Hex” and/or Na methanolate was fed into column A.

As in Examples 1 and 2, 3.4 kg of “Hex” were added per kg of solids and0.05 kg Na/kg of solids (based on solids in vapors a and b).

The results are to be found in Table 3.

TABLE 3 Example 3 XI IX Service BD loss life VIII in Heat I III BDworking X exchanger THF II BD IV V VI VII working up Solid ColumnsWorking-up Reac- THF Discharge a b b + a a up (THF + dis- dischargePipelines variant tion Distill. C or D back in back in a + b charge) Cor D (in days) 3.1. *) 3.5 0.5 3.5 12.5 8.4 — 7.1 15.5 4.0 0.9 15-203.2. 3.5 <0.1 2.3 13.2 8.4 — 7.1 15.5 2.3 0.9  30-200 3.3. 3.5 <0.1 <0.115.5 8.4 — 7.1 15.5 <0.1 0.9  30-150 3.4. *) 3.5 0.75 2.3 12.45 8.4 —7.1 15.5 3.05 0.9  7-10 3.5. *) 3.5 0.75 4.6 10.15 8.4 — 7.1 15.5 5.350.9  7-10 I—X: all data in mol X: one building block of the PBT polymerof diol and ester (molecular weight: 220 g) counts as mol I: THFformation in stages 1 and 2 II: THF formation during working up III: BDin the bottom product discharged from C and D IV: recovered BD fromvapors a and b V: BD in vapors b VII: BD in vapors a VIII: BD fed intothe working up IX: total loss of BD in the working up X: solidsdischarged from C and D (0.1 mol of PBT based on 35.2 mol of DMTemployed) *) for comparison

We claim:
 1. A process for recovering dihydroxy compounds from residueswhich result from the preparation of polyesters, which process comprisesthe steps of esterifying or transesterifying a dicarboxylic acid with amolar excess of a dihydroxy compound in a first stage, wherein anesterification or transesterification product and vapors (a) are formed;polycondensing the esterification or transesterification product, or amixture thereof, in at least one second stage, wherein vapors (b),comprising vapors (b1) and vapors (b2), are formed; transferring vapors(a) and vapors (b1) to at least one first column A; distilling combinedvapors (a) and vapors (b1) in first column A, resulting in low-boilingcomponents as distillate and a bottom product comprising excessdihydroxy compound and oligomeric and polymeric reaction products;transferring the bottom product from column A to the first stage;transferring vapors (b2) to at least one second column B; distillingvapors (b2) in second column B, resulting in low-boiling components asdistillate and a bottom product; transferring the bottom product fromsecond column B to at least one third column C; distilling the bottomproduct from second column B in third column C, resulting in dihydroxycompounds and a bottom product; transferring the bottom product fromthird column C to an evaporation apparatus D; removing dihydroxycompound from the bottom product in apparatus D; and returning dihydroxycompound from apparatus D to third column C; wherein the treatment torecover the dihydroxy compounds is carried out in the presence of analkali metal or alkaline earth metal compound in an amount of from 0.5to 10 % by weight, calculated as alkali earth metal or alkaline earthmetal, based on the solids content of the vapors.
 2. A process forrecovering dihydroxy compounds from residues which result from thepreparation of polyesters, which process comprises the steps ofesterifying or transesterifying a dicarboxylic acid with a molar excessof a dihydroxy compounds in a first stage, wherein an esterification ortransesterification product and vapors (a) are formed; polycondensingthe esterification or transesterification product, or a mixture thereof,in at least one second stage, wherein vapors (b) are formed;transferring vapors (a) to at least one first column A; distillingvapors (a) in first column A, resulting in low-boiling components asdistillate and a bottom product comprising excess dihydroxy compoundsand oligomeric and polymeric reaction products; transferring the bottomproduct from column A to the first stage; transferring vapors (b) to atleast one second column B; distilling vapors (b) in second column B,resulting in low-boiling components as distillate and a bottom product;transferring the bottom product from second column B to at least onethird column C; distilling the bottom product from second column B inthird column C, resulting in dihydroxy compounds and a bottom product;transferring the bottom product from third column C to an evaporationapparatus D; removing dihydroxy compounds from the bottom product inapparatus D; and returning dihydroxy compounds from apparatus D to thirdcolumn c; wherein the treatment to recover dihydroxy compounds iscarried out in the presence of an alkali metal or alkaline earth metalcompounds in an amount of from 0.5 to 10 % by weight, calculated asalkali earth metal or alkaline earth metal, based on the solids contentof the vapors.
 3. A process for recovering dihydroxy compounds fromresidues which result the preparation of polyesters, which processcomprises the steps of esterifying or transesterifying a dicarboxylicacid with a molar excess of a dihydroxy compounds in a first stage,wherein an esterification or transesterification product and vapors (a)are formed; polycondensing the esterification or transesterificationproduct, or a mixture thereof, in at least one second stage, whereinvapors (b) are formed; transferring vapors (a) and vapors (b) to atleast one first column A; distilling vapors (a) and vapors (b) in firstcolumn A, resulting in low-boiling components as distillate and a bottomproduct comprising excess dihydroxy compounds and oligomeric andpolymeric reaction products; transferring the bottom product from firstcolumn A to at least one second column C; distilling the bottom productfrom first column A in second column C, resulting in dihydroxy compoundsand a bottom product; transferring the bottom product from second columnC to an evaporation apparatus D; removing dihydroxy compounds from thebottom product in apparatus D; and returning dihydroxy compounds fromapparatus D to second column C; wherein the treatment to recoverdihydroxy compounds is carried out in the presence of an alkali metal oralkaline earth metal compounds in an amount of from 0.5 to 10 % byweight, calculated as alkali earth metal or alkaline earth metal, basedon the solids content of the vapors.
 4. The process of claim 1 whereinthe alkali metal compound is an alcoholate.
 5. The process of claim 4wherein the alcoholate is sodium or potassium methanolate, or a mixturethereof.
 6. The process of claim 1 wherein the low-boiling component ofthe vapors comprises a mixture of water or methanol with tetrahydrofuranor acetaldehyde.
 7. The process of claim 1 which further comprises thestep of adding a liquid residue comprising a dihydroxy compound to thebottom product in column C in a further treatment to recover dihydroxycompounds.
 8. The process of claim 1 wherein the temperature inapparatus D is above the melting point of the remaining residue.
 9. Theprocess of claim 1 wherein the recovered dihydroxy compound from columnC is returned to the first stage.
 10. The process of claim 2 wherein thealkali metal compound is an alcoholate.
 11. The process of claim 10wherein the alcoholate is sodium or potassium methanolate, or a mixturethereof.
 12. The process of claim 2 wherein the low-boiling component ofthe vapors comprises a mixture of water or methanol with tetrahydrofuranor acetaldehyde.
 13. The process of claim 2 which further comprises thestep of adding a liquid residue comprising a dihydroxy compound to thebottom product in column C in a further treatment to recover dihydroxycompounds.
 14. The process of claim 2 wherein the temperature inapparatus D is above the melting point of the remaining residue.
 15. Theprocess of claim 2 wherein the recovered dihydroxy compound from columnC is returned to the first stage of the process.
 16. The process ofclaim 3 wherein the alkali metal compound is an alcoholate.
 17. Theprocess of claim 16 wherein the alcoholate is sodium or potassiummethanolate, or a mixture thereof.
 18. The process of claim 3 whereinthe low-boiling component of the vapors comprises a mixture of water ofmethanol with tetrahydrofuran or acetaldehyde.
 19. The process of claim3 which further comprises the step of adding a liquid residue comprisinga dihydroxy compound to the bottom product in column C in a furthertreatment to recover dihydroxy compounds.
 20. The process of claim 3wherein the temperature in apparatus D is above the melting point of theremaining residue.
 21. The process of claim 3 wherein the recovereddihydroxy compound from column C is returned to the first stage of theprocess.